Algae, belonging to the class of phototrophic microorganisms, are organisms that can efficiently convert light into biomass using photosynthesis. The photosynthesis process is conversion of light energy into chemical energy by living organisms. The raw materials are carbon dioxide and water; the energy source is light; and the end-products are oxygen and (energy rich) carbohydrates.
Algae and other photosynthetic organisms have been recognized as an efficient producer of biomass, and in particular oil from which biodiesel and other fuels can be produced. During photosynthesis, algae and other photosynthetic organisms absorb carbon dioxide (CO2) and light (photons) in the presence of water and produce oxygen and biomass. Dissolved nutrients may assist the process. Algae and other photosynthetic organisms can produce lipids or vegetable oils which can be harvested and converted into biodiesel and other biofuels or used directly.
The benefits of using algae to efficiently grow biomass and produce biofuel have been known for a long time, and various methods have been used to grow algae in laboratories and small scale experimental units. However, it has proven difficult to grow algae efficiently on a commercial scale.
Open pond systems have been used to grow algae on a large scale. Most of the systems used to cultivate micro-algae are shallow ponds. In these ponds, micro-algae can be cultivated with an efficiency of about 2% of sunlight in the PAR region only. PAR is the photosynthetic active region that can be used by algae and other photosynthetic organisms, i.e. sunlight with a wavelength between 400 and 700 nm. Sunlight is spread over a much broader spectrum, and the energy content of sunlight in the PAR region is only about 43% of the total energy content of sunlight. Algae can theoretically convert about 20% of the collected radiation (within PAR) into biomass. However, in most cases, this efficiency is lower because light is absorbed in a much higher rate than the rate at which photons can be converted into biomass. However, in open pond systems it is difficult to control temperature and pH, and difficult to prevent foreign algae and bacteria from invading the pond and competing with the desired algae culture. Furthermore, much of the sunlight is reflected by the water's surface, and the sunlight that does enter the pond only penetrates a small distance into the water due to the algae becoming so dense and blocking the light, so that the sunlight only reaches a thin layer of algae growing near the surface of the pond.
Bioreactors have also been used, in which nutrient-laden water is pumped through plastic or glass tubes or plates that are exposed to sunlight. Such reactors are for example known from Singh et al, Journal of Applied Phycology 12: 269-275, 2000, from Usui, Energy Conyers. Mgmt, vol. 38, Supple., pages S487-S492, 1997. The photochemical efficiency of photo bioreactor systems, especially of the flat-plate glass reactor, can reach about 16%, which is much higher than for micro-algae in ponds.
However, such bioreactors known from the state of the art have still some disadvantages. Such bioreactors are more costly and more difficult to operate than open pond systems, and they also suffer from the problem of getting the sunlight to the algae where it can be absorbed. A large portion of the sunlight is reflected from the surface of the tubes or plates. Only a small amount of the sunlight enters the water in the tubes or plates, and this small amount of sunlight only penetrates a small distance into the volume of the tube or plate. Other drawbacks of such bioreactor systems are the difficulty of temperature control, and the reliance on sunlight for growing the culture.
Algae grow best under controlled conditions. Algae is sensitive to temperature and light conditions. By controlling all aspects of the cultivation, such as temperature, CO2 levels, light and nutrients, extremely high yields can be obtained.
A reactor for a photosynthetic culture is also known from Kondo et al., U.S. Pat. No. 6,287,852. A disadvantage of this reactor is the use of fixed collectors, which means that during most of the time, radiation of the sun is not collected efficiently.
WO2005068605 describes a reactor for cultivating phototrophic micro organisms, wherein the sunlight is introduced in compartment walls by using one or more moveable collimators. The compartment walls are transparent and from there light is distributed into the reactor. Such a reactor has an improved collection of radiation and an improved distribution of the radiation into the reactor, thereby providing a more efficient reactor and a more efficient cultivation of phototrophic micro organisms.